Motor speed detecting device

ABSTRACT

Disclosed is a motor speed detecting device which comprises a first slotted disc (3) having one slot and mounted on a rotational shaft (2) of a motor (1) that is driven by a motor drive unit (7) and a second disc (4) having a plurality of slots, a first magnetic sensor (5) mounted on the first gear (3) in close proximity thereto and outputting two signals with a phase difference and a second magnetic sensor (6) mounted on the second slotted disc in close proximity thereto and outputting two signals with the phase difference and their inverted signals and a detector circuit (8) receiving the output of the first and the second magnetic sensors. The detector circuit (8) is provided, wherein two outputs from the first magnetic sensor are compared with a differential amplified voltage and a reference voltage, one pulse per one revolution of the rotational shaft (2) is output, and in addition, with the four outputs provided from the second magnetic sensor (6), differential amplification, inverting amplification, comparison and vector synthetic comparison are implemented to provide a pulse per 22.5 degrees, where the phase of the output waveform in the magnetic sensor in response to the width of the slot, resulting in a sinusoidal wave having a different phase by 90 degrees from each other, in response to the number of the slots of the second disc (4), and a square wave of as many as four times the frequencies as the number of slots of the second disc (4) and a square wave of one-fourth period of phase difference with regard to the square wave, are provided to output therewith. Thus, using the relevant device, it is applicable to a to-be-detected rotational shaft of which the rotational shaft (2) is hollow and to provide a precision of detection four times as great.

TECHNICAL FIELD

The present invention relates to a motor speed detecting device, andmore particularly, to a motor speed detection device utilizing amagnetic sensor and a slotted disc having an undercut notched wedge andmounted on the rotational shaft of the motor.

BACKGROUND ART

In the prior art, a motor speed detecting device comprises a coded disccomposed of a glass plate mounted on the rotational shaft of the motor,and as an opaque portion of the coded disc passes between a light sourceand a light sensor, an electrical signal is obtained from the lightsensor. The electric signal is then amplified to shape the waveform,thus providing a series of pulses in response to the rotational speed ofthe shaft. Usually, this device outputs two types of pulses, that is tosay, one pulse per one revolution of the coded disc and a plurality ofpulses per one revolution of the coded disc. The device counts thenumber per hour of these pulses, and thus detects the rotational speedof the motor.

There is an unavoidable problem wherein, when the detected rotationalshaft is hollow, the above-described speed detection device cannot beapplied. When a motor is used for driving a machine tool such as a latheand the like, it is necessary to render a drive rotational shaft hollowand to make a cylindrical workpiece pass therethrough, on the basis ofthe mechanism for delivering the workpiece. In such a case, theapparatus using the coded disc cannot be adapted from the structuralpoint of view, although it is recognized that the interference fromother sources of light considerably affects for the worse the apparatususing the coded disc, and there remains a problem in that the precisionof the device cannot always be maintained.

The present invention is intended to solve the problems described in theabove prior art devices.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a speed detectiondevice that, when used for a hollow rotational shaft such as in a lathe,it is possible to improve the precision of detection in response to thenumber of slots of the slotted disc by approximately four times withoutincreasing the number of slots.

According to a feature of the present invention, there is provided amotor speed detecting device comprising, a first disc composed offerromagnetic material, having one slot and mounted on the rotationalshaft of a motor driven by a motor drive unit; a second disc composed offerromagnetic material, having a plurality of slots; a first magneticsensor mounted in close proximity to the first disc, for outputting twosignals having a phase difference responsive to the width of the notcheddisc by one half; a second magnetic sensor mounted in close proximity tothe second disc, for outputting two signals having the phase differenceresponsive to the width of the slot by one half, and two other signalsin phase with the signals inverted by the first two signals; and adetector circuit for receiving the outputs of the first and secondmagnetic sensors and detecting those outputs.

The detector circuit comprises, a first differential amplifier circuitfor counting the difference between the two signals from the firstmagnetic sensor, a first comparator circuit for comparing the outputfrom the first differential amplifier circuit with a reference voltage,a second differential amplifier circuit and a third differentialamplifier circuit for receiving respectively two sets of signals havingan opposite phase relationship to each other from the second magneticsensor and counting the difference therebetween, a vector syntheticcomparator circuit for receiving the outputs of the second and thirddifferential amplifier circuit and searching for the signals having eachphase of 0, 22.5, 45, 67.5, 90, 112.5, 135 and 157.5 degrees, at a phasedifference of 22.5 degrees, separately, and when the phase angle of theoutput waveform of the magnetic sensor in response to the width of theslot is 180 degrees, then comparing the signals with the referencevoltage to make a series of eight rectangular waves, a logic circuit foroperating the output of the vector synthetic comparator circuitlogically, a first inverting amplifier circuit for inverting andamplifying the output of the second differential amplifier circuit, anda second inverting amplifier circuit for inverting and amplifying theoutput of the third differential amplifier circuit, wherein thesinusoidal wave output of the first and second inverting amplifiercircuit is rendered to control the motor through the motor drive unitand to provide one pulse per one revolution of the rotational shaft tothe output of the first comparator circuit, to provide a rectangularwave four times as large as the frequency in response to the number ofslots of the second disc to the output of the logic circuit and toprovide a rectangular wave having a phase difference of the period byone quarter to the rectangular wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the constitution of a motor speed detectingdevice according to an embodiment of the present invention;

FIG. 2 is a diagram showing a slotted disc and magnetic sensor portionin FIG. 1;

FIG. 3 is a diagram showing the first slotted disc and the firstmagnetic sensor in FIG. 2;

FIG. 4 is a diagram showing the second slotted disc and the secondmagnetic sensor in FIG. 2;

FIG. 5 is a waveform diagram showing the output waveforms of themagnetic sensor in FIG. 2;

FIGS. 6A and 6B are block circuit diagrams of each detector circuit inFIG. 1;

FIG. 7 is a circuit diagram of a vector synthetic comparator in FIGS. 6Aand 6B; and

FIG. 8 is a waveform diagram showing the waveforms of signals in thecircuits of FIGS. 6A and 6B.

BEST MODE FOR CARRYING OUT THE INVENTION

The constitution of the motor speed detecting device and the slotteddisc and magnetic sensor portion thereof are shown respectively in FIG.1 and FIG. 2 in accordance with an embodiment of the present invention.The device comprises a motor 1 having a rotational shaft 2, a firstslotted disc 3, a second slotted disc 4, a first magnetic sensor 5, asecond magnetic sensor 6, a detector circuit 8, and a motor drive unit7.

The discs 3 and 4 are composed of ferromagnetic materials and are fixedto the rotational shaft 2. A slot 31 is cut in the disc 3, as shown inFIG. 3. The magnetic sensor 5 is mounted opposite to the disc 3. Themagnetic sensor 5 contains a magnetic resistance effect element (denotedby a resistor symbol in the figure) and a permanent magnet (not shown).When the width of the slot 31 is taken as λ, the magnetic resistanceeffect element is arranged in the peripheral direction of the disc 3 bya spacing of λ/2. The permanent magnet is disposed in relation to thedisc 3 to sandwich therebetween the magnetic resistance effect element.When the slot of the disc 3 is opposite the permanent magnet, it islikely to weaken a magnetic field that is provided to the magneticresistance effect element. The magnetic resistance effect element hasthe property of varying the value of an electric resistance in responseto the strength of the magnetic field. Consequently, when the magneticresistance effect element is connected in a way such as shown in FIG. 3,and applied to a power source, and the slot of the disc 3 passes by inclose proximity to the magnetic sensor 5, both outputs Z and Z of themagnetic sensor 5 have a signal waveform as shown in the waveform (1) ofFIG. 5.

The disc 4 is provided with a plurality of slots as shown in FIG. 4. Tosimplify the explanation, in the figure the width of the slot is shownenlarged and the slots arranged uniformally around the periphery areshown by chain lines. The magnetic sensor 6 comprises two magneticresistance effect elements, in comparison with the magnetic sensor 5 ofFIG. 3, to provide the inverted outputs having phase angles different by180 degrees A and B of the A and B outputs, other than A and B outputsin response to the Z and Z outputs of the magnetic sensor 5. If thewidth of wedge is λ, as shown by the resistance symbol in FIG. 4, in theset of elements providing A and B outputs each element is arranged witha spacing of λ/2, in a set of elements providing A and B outputs eachare arranged, for instance, with a spacing of λ/2, and the power sourceis connected in opposite polarity therewith, to be further arranged witha spacing of 2λ to a set of the elements. In this way, with the outputof the magnetic sensor 6, an output is provided such as the waveforms(2) and (3) as shown in FIG. 5. The passage of the disc 4 in theneighbourhood of a permanent magnet causes the magnetic field to vary;this is the same operational principle, in which the resistance value ofthe magnetic resistance effect element is changed to provide an outputsignal, as that in the case of the disc 3 and the magnetic sensor 5.

A block circuit diagram of the detector circuit 8 is shown in FIG. 6Aand FIG. 6B. The outputs Z and Z from the magnetic sensor 5 are suppliedto a differential amplifier 813 as the first differential amplifiercircuit to provide the difference signal. The difference signal iscompared with a reference voltage, for instance, 2.5 V, in a comparator849 as the first comparator circuit, so one rectangular wave per onerevolution of the disc is output through a driver 873.

The outputs A and A from the magnetic sensor 6 are applied to adifferential amplifier 811 as the second differential amplifier circuitto count the difference, and are then supplied to a vector syntheticcomparator 846 to 848 forming a vector synthetic comparator circuit,through a comparator 841, vector synthetic comparators 842 to 844forming a vector synthetic comparator circuit, and an inverter 831. Theoutputs B and B from the magnetic sensor 6 are applied to a differentialamplifier 812 as the third differential amplifier circuit to count thedifference, and are then supplied to a comparator 845 and vectorsynthetic comparators 842 through 844, and 846 through 848 forming thevector synthetic comparator circuit.

Each vector synthetic comparator 842, 843, 844, 846, 847, and 848comprises an operational amplifier 91 and resistors respectively asshown in FIG. 7. The resistance values differ in each vector syntheticcomparator. Considering the phase of the output signal S' (811) of thedifferential amplifier 811 as a reference, as the output signal S (812)of the differential amplifier 812 has a phase difference of 90 degrees,these two signals S (811) and S (812) are added with a proper amplituderatio to provide a signal of a desired phase difference. That is to say,when the signal S (811) of a phase difference of 0 degrees is applied toan input terminal IN1, and the signal S (812) of a phase difference of90 degrees to an input terminal IN2, as shown in FIG. 7, then if theresistance of resistor R1 and the resistance of resistor R2 are selectedto satisfy the equation:

    R1/R2=tan 22.5°

a signal having a phase difference of 22.5 degrees is applied to theinput of the operational amplifier 91 and compared with a referencevoltage (V_(ref)), for instance, 2.5 V, to output the rectangular signalhaving a phase difference of 22.5 degrees. A vector synthetic comparator843 selects the resistor R1 and R2 so as to satisfy the equation:

    R1/R2=tan 45°=1

to provide a rectangular wave signal having a phase difference of 45degrees at the output. On the other hand, a vector synthetic comparator844 selects a resistor R1 and R2 so as to satisfy the equation:

    R1/R2=tan 67.5°

to provide a rectangular wave signal having a phase difference of 67.5degrees at the output.

The signal S (812) is applied to the input IN1 of the vector syntheticcomparator 846 and an inverted signal of the signal S (811) to the inputIN2 through an inverter 831, so the resistors R1 and R2 are selected soas to satisfy the equation;

    R2/R1=|tan 112.5°|

to provide a rectangular wave signal having a phase difference of 112.5degrees at the output. Similarly a vector synthetic comparator 847selects resistors R1 and R2 so as to satisfy the equation:

    R2/R1=|tan 135°|=1

to provide a rectangular wave signal having a difference of 135 degrees.A vector synthetic comparator 848 selects resistors R1 and R2 so as tosatisfy the equation:

    R2/R1=|tan 157.5°|

to provide a rectangular wave having a phase difference of 157.5 degreesat the output. The signal S (811) is applied to the comparator 841, andcompared with the reference voltage, to output a rectangular wave havinga phase difference of zero degrees, while the signal S (812) is suppliedto the comparator 845, to output a rectangular wave having a phasedifference of 90 degrees.

If the output signals of the above-mentioned vector synthetic comparator842 to 844 are respectively S (842) to S (844), the output signals ofthe vector synthetic comparator 846 to 848, respectively S (846) to S(848), and the output signals of the comparator 841 and 845,respectively S (841) and S (845), the waveforms of the signals S (841)to S (848) are as shown in FIG. 8.

The signals S (841) and S (843) are applied to the exclusive OR gate 851to provide an output signal S (851). The signals (842) and S (844) areapplied to the exclusive OR gate 852 to provide an output signal S(852). The signals S (845) and S (847) are applied to the exclusive ORgate 813 to provide an output signal S (853). The signals S (846) and S(848) are applied to the exclusive OR gate 854 to provide an outputsignal (854).

The signals S (851) and S (853) are applied to the OR gate 861 toprovide the output signals S (861). The signals (852) and S (854) areapplied to the OR gate 862 to provide the output signal S (862). Thewaveforms of the above-mentioned signals S (851) to S (854), S (861) andS (862) are as shown in FIG. 8.

If the signal S (861) is compared with the signal S (84) derived fromthe signals A and A, the frequency is four times larger and there is aphase difference of 90 degrees between the signal S (861) and the signalS (862). The signals S (861) and S (862) are supplied respectivelythrough the driver 871 and 872 as the output of the detector circuit 8.Additionally, the signal S (811) is output through the invertingamplifier 821 as the first inverting amplifier circuit to provide thesinusoidal wavesignal to the above-mentioned motor drive unit 7, whilethe signal S (812) is output through the inverting amplifier 822 as thesecond inverting amplifier circuit to provide the sinusoidal wave signalto the motor drive unit 7 in the same manner.

As mentioned above, according to an embodiment of the present invention,there is provided one pulse per one revolution of the rotational shaftof the motor, the sinusoidal wave signal per one revolution beingequivalent to the number of slots of the slotted disc having a pluralityof slots mounted on the rotational shaft, the sinusoidal wave signalhaving the phase difference of 90 degrees to the sinusoidal signal andthe rectangular wave signals having the phase difference of 90 degreesto each other at four times the frequencies of the sinusoidal signals.

We claim:
 1. A motor speed detecting device comprising:a first slotteddisc composed of ferromagnetic material, having one slot and beingmounted on the rotational shaft of a motor driven by a motor drive unit;a second slotted disc composed of ferromagnetic material, having aplurality of slots; a first magnetic sensor mounted in close proximityto said first slotted disc, for outputting a first pair signals having aphase difference in response to the width of said slot by one half; asecond magnetic sensor mounted in close proximity to said second slotteddisc, for outputting a second pair of signals having a phase differencein response to the width of said slot by one half, and also foroutputting a third pair of signals inverted with respect to said secondpair of signals; and a detector circuit for receiving the outputs ofsaid first and second magnetic sensors and for detecting said outputs;said detector circuit comprising: a first differential amplifier circuitfor counting the difference between said first pair of signals from saidfirst magnetic sensor; a first comparator circuit for comparing theoutput from said first differential amplifier circuit with a referencevoltage; a second differential amplifier circuit and a thirddifferential amplifier circuit for receiving respectively said secondand third pairs of signals having an opposite phase relationship to eachother from said second magnetic sensor and counting the differencetherebetween; a vector synthetic comparator circuit for receiving theoutputs of said second and third differential amplifier circuits andsearching for the signals having each phase angle of 0, 22.5, 45, 67.5,90, 112.5, 135 and 157.5 degrees at a phase difference of 22.5 degreesrespectively, when the phase angle of the output waveform of themagnetic sensor in response to the width of said slot is 180 degrees,then comparing said signals with the reference voltage to make a seriesof eight rectangular waves; a logic circuit for operating the output ofsaid vector synthetic comparator circuit logically; a first invertingamplifier circuit for inverting and amplifying the output of said seconddifferential amplifier circuit; and a second inverting amplifier circuitfor inverting and amplifying the output of said third differentialamplifier circuit; wherein the sinusoidal wave outputs of said first andsecond inverting amplifier circuit are rendered to control said motorthrough said motor drive unit and to provide one pulse per onerevolution of said rotational shaft to the output of said firstcomparator circuit, to provide a rectangular wave of the frequency fourtimes as large as the frequency corresponding to the number of slots ofsaid second disc to the output of said logic circuit and to provide arectangular wave having a one quarter period of phase difference fromsaid rectangular wave.